The subject matter disclosed herein relates to gas appliances, such as gas ranges, and more particularly, to gas burner assemblies for use in such gas appliances.
Atmospheric gas burners are often used as surface units in household gas cooking appliances. A significant factor in the performance of gas burners is their ability to withstand airflow disturbances in the surroundings, such as room drafts, rapid movement of cabinet doors, and rapid oven door manipulation. Manipulation of the oven door is particularly troublesome because rapid openings and closings of the oven door often produce respective under-pressure and over-pressure conditions within the oven cavity. Since the flue, through which combustion products are removed from the oven, is sized to maintain complete combustion of gaseous fuels and is generally inadequate to supply a sufficient air flow for re-equilibration, a large amount of air passes through or around the gas burners. In particular, pressure fluctuations from, for example, cabinet or door openings, cause the structures to expand or contract (e.g., the sheet metal deflects) and this structural movement pumps air into adjacent cavities, causing the temporary under or over pressure conditions. This surge of air around the gas burners, due to over pressure or under pressure conditions in the oven cavity, is detrimental to the flame stability of the burners and may extinguish the flames. This flame stability problem is particularly evident in sealed gas burner arrangements, referring to the lack of an opening in the cooktop surface around the base of the burner to prevent spills from entering the area beneath the cooktop.
A cause of flame instability is the low pressure drop of the fuel/air mixture passing through the burner ports of a typical rangetop burner. Although there is ample pressure available in the fuel, the pressure energy is used to accelerate the fuel to the high injection velocity required for primary air entrainment. Relatively little of this pressure is recovered at the burner ports. A low pressure drop across the ports allows pressure disturbances propagating through the ambient to easily pass through the ports, momentarily drawing or pulling the flame away from the burner ports and leading to thermal quenching and extinction.
An additional problem is that rapid adjustments of the fuel supply to a gas burner from a high burner input rate to a low burner input rate often will cause flame extinction when the momentum of the entrained air flow continues into the burner even though fuel has been cut back, resulting in a momentary drop in the fuel/air ratio, causing extinction.
A number of techniques have been proposed or suggested for improving stability performance. U.S. Pat. No. 5,133,658, for example, employs an expansion chamber to improve flame stability. The disclosed gas burners have a plenum ahead of a number of main burner ports. An expansion chamber inlet is located in the plenum, adjacent the main flame ports. When a pressure disturbance enters the burner (suction, for example, from the opening of an oven door), the pressure drop and flow velocity through the main burner ports are momentarily disrupted causing unwanted extinction of the main burner flames. The expansion chamber flame, however, is less susceptible to extinction due to the damping effect described in earlier art. Although such gas burners having an expansion chamber provide somewhat improved stability performance at simmer settings, disturbances continue to cause unwanted extinction. Furthermore, these expansion chambers have excessively large flames at higher burner input rates.
U.S. Pat. No. 5,800,159 to Maughan et al. (hereinafter, the “'159 Patent”) overcomes the issue of excessively large flames using a stability chamber that is insensitive to input rates. The '159 Patent discloses an improved chamber where the inlet ports that feed the stability chamber are located substantially near the Venturi throat such that the volume of gas entering the chamber at higher flow rates is disproportionally low relative to the volume of gas entering the chamber at lower flow rates. Generally, the techniques of the '159 Patent seek to limit the flame length of the stability chamber at higher flow rates when the stability chamber is not needed. However, one inherent weakness of the approach disclosed in the '159 Patent is the need for physically larger inlet ports to the chamber than is traditionally needed with conventional stability chambers in order to get sufficient gas flow into the chamber at low flow rates. Larger ports with lower pressure drops (and thus lower velocities of gas traveling through them) have an increased tendency to flashback. In addition, since the scale of a stability chamber is limited (they cannot be reduced in size below a minimum volume needed to support a single flame without withdrawing too much heat from the flame into the boundary walls, thereby quenching the flame), the effects of the disclosed teachings become exaggerated in smaller burner sizes, making it troublesome to optimize smaller burner designs without the need for even larger chamber inlet port sizes.
A further problem with the approach disclosed in the '159 Patent is realized when burner caps use projections to position themselves concentrically on the burners. Caps made of stamped steel offer cost advantages to alternative caps such as sintered metal or die cast forms. Welding studs to the bottom side gives low cost stamped steel caps a means to be positioned and, if done correctly, will be obvious to the user when assembled onto the burner incorrectly. These projections can often be positioned unintentionally into the chamber since the chamber travels the bulk of the radial length of the burner. When this happens, the cap may be positioned in an undesirable, non-concentric state. Additional projections or studs may be added to a cap to overcome this problem, but this increases the cost of the caps and interference of fuel flow through the burner head.
Thus, there remains a need for an improved atmospheric gas burner that is better able to withstand flashback tendencies. Yet another need exists for stability chambers that can better prevent miss-assembly of caps that use projections to position themselves, while also taking advantage of the teachings of the '159 Patent.